一,一对一
一对一映射是指一个对象与另一个对象具有一对一的关系
1.1 表关系
一对一映射是指一个对象与另一个对象具有一对一的关系。例如,一个用户(User)与一个地址(Address)之间的关系。假设我们有以下表结构:
用户表:
id (int)
name (varchar)
address_id (int)
地址表:
id (int)
street (varchar)
city (varchar)
首先,创建 User 和 Address 实体类:
User.class
public class User {
private int id;
private String name;
private Address address;
// getters and setters
}
Address.class
public class Address {
private int id;
private String street;
private String city;
// getters and setters
}
接下来,创建一个 resultMap 进行一对一映射:
1.2 resultMap设置自定义映射
UserMapper.xml
<resultMap id="UserAddressResultMap" type="User">
<id property="id" column="id"/>
<result property="name" column="name"/>
//一对一关系使用association
<association property="address" javaType="Address">
<id property="id" column="address_id"/>
<result property="street" column="street"/>
<result property="city" column="city"/>
</association>
</resultMap>
属性:
id:表示自定义映射的唯一标识
type:查询的数据要映射的实体类的类型
子标签:
id:设置主键的映射关系
result:设置普通字段的映射关系
association :设置多对一的映射关系
collection:设置一对多的映射关系
子标签属性:
property:设置映射关系中实体类中的属性名
column:设置映射关系中表中的字段名
最后,编写一个查询方法来使用这个 resultMap:
UserMapper.xml
<select id="findUserWithAddress" resultMap="UserAddressResultMap">
SELECT u.id, u.name, a.id as address_id, a.street, a.city
FROM user u
INNER JOIN address a ON u.address_id = a.id
WHERE u.id = #{id}
</select>
最后实现接口findUserWithAddress方法测试即可,通过以上简单的案例,我相信你已经明白自定义关系映射了。往往实际开发中数据和表是要复杂的多,进阶用法请看以下示例:
二、一对多映射(One-to-Many)
一对多映射是指一个对象与多个对象具有一对多的关系。例如,一个订单(Oeder)与一个订单详情(OrderItem)之间的关系。假设我们有以下表结构:
2.1 创建实体
利用mybatis逆向工程(generatorConfig.xml)生成模型层代码 :
package com.xzs.model;
import lombok.ToString;
@ToString
public class order {
private Integer orderId;
private String orderNo;
public order(Integer orderId, String orderNo) {
this.orderId = orderId;
this.orderNo = orderNo;
}
public order() {
super();
}
public Integer getOrderId() {
return orderId;
}
public void setOrderId(Integer orderId) {
this.orderId = orderId;
}
public String getOrderNo() {
return orderNo;
}
public void setOrderNo(String orderNo) {
this.orderNo = orderNo;
}
}
package com.xzs.model;
import lombok.ToString;
@ToString
public class orderItem {
private Integer orderItemId;
private Integer productId;
private Integer quantity;
private Integer oid;
public orderItem(Integer orderItemId, Integer productId, Integer quantity, Integer oid) {
this.orderItemId = orderItemId;
this.productId = productId;
this.quantity = quantity;
this.oid = oid;
}
public orderItem() {
super();
}
public Integer getOrderItemId() {
return orderItemId;
}
public void setOrderItemId(Integer orderItemId) {
this.orderItemId = orderItemId;
}
public Integer getProductId() {
return productId;
}
public void setProductId(Integer productId) {
this.productId = productId;
}
public Integer getQuantity() {
return quantity;
}
public void setQuantity(Integer quantity) {
this.quantity = quantity;
}
public Integer getOid() {
return oid;
}
public void setOid(Integer oid) {
this.oid = oid;
}
}
创建 OrderVo
它是一个值对象(Value Object),继承 Order
类并添加了一个额外的属性 orderItems
。该类用于在应用程序的各个层之间传递数据,尤其是在表示层和业务逻辑层之间。在这个例子中,OrderVo
类用于表示一个订单及其关联的订单项。
package com.xzs.vo;
import com.xzs.model.Order;
import com.xzs.model.OrderItem;
import lombok.Data;
import java.util.ArrayList;
import java.util.List;
@Data
public class OrderVo extends Order {
private List<OrderItem> orderItems = new ArrayList<>();
}
2.2 级联方式处理映射关系
<resultMap id="OrderVoMap" type="com.xzs.vo.OrderVo">
<result column="order_id" property="orderId"></result>
<result column="order_no" property="orderNo"></result>
//多关系使用collection
<collection property="orderItems" ofType="com.xzs.model.OrderItem">
<result column="order_item_id" property="orderItemId"></result>
<result column="product_id" property="productId"></result>
<result column="quantity" property="quantity"></result>
<result column="oid" property="oid"></result>
</collection>
</resultMap>
2.3 定义SQL
<select id="SelectByOid" resultMap="OrderVoMap" parameterType="java.lang.Integer">
SELECT *
FROM t_hibernate_order o,
t_hibernate_order_item oi
WHERE o.order_id = oi.oid
AND o.order_id = #{oid}
</select>
2.4 OrderMapper接口
package com.xzs.mapper;
import com.xzs.model.Order;
import com.xzs.vo.OrderVo;
import org.apache.ibatis.annotations.Param;
import org.springframework.stereotype.Repository;
@Repository
public interface OrderMapper {
OrderVo SelectByOid(@Param("oid") Integer oid);
}
2.5 编写业务逻辑层
OrderItmeBiz 接口
package com.xzs.biz;
import com.xzs.vo.OrderItemVo;
public interface OrderItmeBiz {
OrderItemVo SelectByOitemId(Integer oid);
}
OrderBizImpl 接口实现类
package com.xzs.biz.impl;
import com.xzs.biz.OrderBiz;
import com.xzs.mapper.OrderMapper;
import com.xzs.vo.OrderVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class OrderBizImpl implements OrderBiz {
@Autowired
private OrderMapper orderMapper;
@Override
public OrderVo SelectByOid(Integer oid) {
return orderMapper.SelectByOid(oid);
}
}
三、多对多映射(Many-to-Many)
3.1 表关系
多对多映射是指多个对象与多个对象具有多对多的关系。表之间的多对多关系稍微复杂,需要一个中间表来表示:
中间表有三个字段,其中两个字段分别作为外键指向各自一方的主键,由此通过中间表来表示多对多关系,通过一个表联合另一个中间表可以表示为一对多关系。
如:根据书籍id查找关联属性类别:
3.2 创建实体
利用mybatis逆向工程(generatorConfig.xml)生成模型层代码 :
package com.xzs.model;
import lombok.ToString;
@ToString
public class HCategory {
private Integer categoryId;
private String categoryName;
public HCategory(Integer categoryId, String categoryName) {
this.categoryId = categoryId;
this.categoryName = categoryName;
}
public HCategory() {
super();
}
public Integer getCategoryId() {
return categoryId;
}
public void setCategoryId(Integer categoryId) {
this.categoryId = categoryId;
}
public String getCategoryName() {
return categoryName;
}
public void setCategoryName(String categoryName) {
this.categoryName = categoryName;
}
}
package com.xzs.model;
import lombok.ToString;
@ToString
public class HBookCategory {
private Integer bcid;
private Integer bid;
private Integer cid;
public HBookCategory(Integer bcid, Integer bid, Integer cid) {
this.bcid = bcid;
this.bid = bid;
this.cid = cid;
}
public HBookCategory() {
super();
}
public Integer getBcid() {
return bcid;
}
public void setBcid(Integer bcid) {
this.bcid = bcid;
}
public Integer getBid() {
return bid;
}
public void setBid(Integer bid) {
this.bid = bid;
}
public Integer getCid() {
return cid;
}
public void setCid(Integer cid) {
this.cid = cid;
}
}
package com.xzs.model;
import lombok.ToString;
@ToString
public class HBook {
private Integer bookId;
private String bookName;
private Float price;
public HBook(Integer bookId, String bookName, Float price) {
this.bookId = bookId;
this.bookName = bookName;
this.price = price;
}
public HBook() {
super();
}
public Integer getBookId() {
return bookId;
}
public void setBookId(Integer bookId) {
this.bookId = bookId;
}
public String getBookName() {
return bookName;
}
public void setBookName(String bookName) {
this.bookName = bookName;
}
public Float getPrice() {
return price;
}
public void setPrice(Float price) {
this.price = price;
}
}
创建 HBookVo 值对象(Value Object)
package com.xzs.vo;
import com.xzs.model.BookCategory;
import com.xzs.model.HBook;
import lombok.Data;
import java.util.List;
@Data
public class HBookVo extends HBook {
private List<BookCategory> bookc = new ArrayList<>();
}
3.3 处理映射关系、定义sql
<resultMap id="HBookVoMap" type="com.xzs.vo.HBookVo" >
<result column="book_id" property="bookId"></result>
<result column="book_name" property="bookName"></result>
<result column="price" property="price"></result>
<collection property="bookc" ofType="com.xzs.model.Category">
<result column="category_id" property="categoryId"></result>
<result column="category_name" property="categoryName"></result>
</collection>
</resultMap>
<!--根据书籍的id查询书籍的信息及所属属性-->
<select id="selectByBookId" resultMap="HBookVoMap" parameterType="java.lang.Integer">
SELECT
*
FROM
t_hibernate_book b,
t_hibernate_category c,
t_hibernate_book_category bc
WHERE
b.book_id = bc.bid
AND c.category_id = bc.bcid
AND b.book_id = #{bid}
</select>
3.4 HBookMapper 接口
HBookVo selectByBookId(@Param("bid") Integer bid);
3.5 编写业务逻辑层
HBookBiz 接口
package com.xzs.biz;
import com.xzs.vo.HBookVo;
public interface HBookBiz {
HBookVo selectByBookId(Integer bid);
}
HBookBizImpl 接口实现类
package com.xzs.biz.impl;
import com.xzs.biz.HBookBiz;
import com.xzs.mapper.HBookMapper;
import com.xzs.vo.HBookVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class HBookBizImpl implements HBookBiz {
@Autowired
private HBookMapper hBookMapper;
@Override
public HBookVo selectByBookId(Integer bid) {
return hBookMapper.selectByBookId(bid);
}
}